Heat Store Dilema - Dropping C/H temp.

I don't have much experience with these things, but my understanding is that you shouldn't run UFH at 75C because of the risk of scalding, so it should be blended down at the UFH manifold. There is typically an additional pump to move water through the blending valve, because there is some recirculation of the water in the UFH (in other words it doesn't all return to the boiler, some of it goes around again, mixed with fresh hot water. There isn't a cold water input like in a shower, the 'cold' comes from the return loop).

AIUI you need that pump to pull in both hot and 'cold' through the mixer, because if you tried to push from the boiler end it wouldn't draw any water through the cold side.

If you want to run the boiler output at 55C, could you perhaps put the thermostatic blending on the output of the TS, ie before the UFH manifold? Then you're distributing 55C water around the place, which is suitable for all your emitters. When you run your back boiler the input to the TS will be hotter (and so the TS will store more energy) but the mixer will make sure the emitter temp doesn't get too high. You'd have to do the calcs to work out if the heat storage at 55C would be enough. (I don't recall why you went for a TS in the first place - was it intermittent heat sources like the back boiler or solar?)

The problem with a thermal store and a low temperature heat input (highly modulated boiler, heat pump, etc) is there's always some deltaT between the heat of the thermal store and the output of the hot water loop - means you can't modulate down to the most efficient level. It's likely not too much of a problem for a boiler, as long as you're prepared to run things that deltaT hotter (which you might need to if you want to store more heat).

Pass. I suppose you still need a pump for the cold side of the mixing valve, but I don't know if the one in the boiler would be suitable.

Not sure, I'm not very familiar with this area and probably missed something.

Theo

Indeed, I have 2 different UFH manifolds one old and one added for the 3 room extension 5 years ago. Both do their own blending with pumps and mixing valve but should the thermostat/mixing valve go dodgy I wouldn't want to supply them with 70 degree water to go around the UFH zones. Original zones are under chipboard/engineered wood and new zones are pipe in screed.

The TS has to be maintained at 75 degrees to give enough heat transfer to DHW which comes from rising main, through a coil in the top part of the store then out through a blending valve to take it back down to 60 degrees so reducing the store temp would reduce DHW "capacity".

I went for a thermal store as I'm adding a smallish wood burner with boiler (4.9kW to water, 3.4kW to room) to the lounge which I'm thinking if the gas boiler has bought the house up to temp by early evening the wood burner may well manage to maintain temp. for most of the evening; plus I have the option of adding Solar Thermal for summer DHW. And... I'm fed-up with the "all eggs in one basket" of a condensing combi-boiler that I regularly had to repair more-so into it's older years.

After re-visiting things today, I'd forgotten the extra pump. The TS has